Besides updating Yaifo for OpenBSD 4.2, I also made the install complete message warn users to use the reboot command instead of typing halt (which could be disastrous if you don't have console access):

CONGRATULATIONS! Your OpenBSD upgrade has been successfully completed!To boot the new system, enter reboot at the command prompt. Once thesystem has rebooted, you will be able to use your system.

Note: Using the halt command at this point may render your installationunreachable. Make sure to use the reboot command instead.